1. Field of the Invention
The present invention relates to a method and an apparatus for controlling absorption of delay fluctuation of data transmitted through a transmission path. The present invention more particularly relates to a method of individually controlling absorption of delay fluctuation of data transmitted through each transmission path and a multiplexer utilizing the method.
2. Description of the Related Art
Recently, a network utilizing a multiplex transmission of data such as images and sounds is found in an increasing number of areas. A type of a multiplexer used in such a network is moving from a time-division multiplexer (TDM) to a cell multiplexer utilizing a statistical multiplex. The time-division multiplexer includes a plurality of time slots in a frame, each timeslot being used for transmitting or receiving corresponding data. Such a time-division multiplexer is suitable for a periodic transmission or reception of information having a single destination address. In a network system such as a company-wide network system where sound quality is important, there is a demand for development of a cell multiplexer whose performance of transmitting or receiving a real-time sound signal is equal to that of the time-division multiplexer. However, in a cell multiplex system, there is a case in which delay fluctuation occurs at a cell multiplexer on a receiving end because of difference in the number of cells transmitted from a cell multiplexer on a transmitting end and in routes of transmitting cells. Such delay fluctuation is a cause of poor sound quality. Thus, the cell multiplexer necessarily includes a buffer for absorbing delay fluctuation of received cells.
FIG. 1 is a diagram showing a sound-signal transmission system including different paths for transmitting sound signals. The sound-signal transmission system shown in FIG. 1 includes cell multiplexers 3, 5, 7 and 10, transmission paths 4, 6, 8 and 9, private branch exchanges (PBX) 2 and 11, and telephone sets 1 and 12. In a case of transmitting an analog sound signal from the telephone set 1 to the telephone set 12, the PBX 2 converts the analog sound signal received from the telephone set 1 to a digital sound signal, and supplies the digital sound signal to the cell multiplexer 3. The cell multiplexer 3 executes a sound compression process and a cell assembly process on the digital sound signal supplied from the PBX 2, and selects a route used for transmitting the digital sound signal made into cells by following exchange information set in a exchange management setting table and speed information set in a scheduler.
In a case of selecting a sound transmission path 1 for transmitting the cells, the cell multiplexer 3 transmits the cells to the cell multiplexer 10 through the transmission path 9. On the other hand, in a case of selecting a sound transmission path 2, the cell multiplexer 3 transmits the cells to the cell multiplexer 10 through the transmission path 4, the cell multiplexer 5, the transmission path 6, the cell multiplexer 7 and the transmission path 8. At the cell multiplexers 3, 5, 7 and 10, delay fluctuation occurs on the cells based on internal processes of the cell multiplexers. Additionally, the transmission paths 4, 6, 8 and 9 cause delay fluctuation on the cells according to conditions of the transmission paths. By setting an amount of the delay fluctuation caused by the cell multiplexers 3, 5, 7 and 10 to “α”, and an amount of the delay fluctuation caused by the transmission paths 4, 6, 8 and 9 to “β” a total amount of the delay fluctuation caused by the sound transmission path 1 becomes “2α+β”. In addition, a total amount of the delay fluctuation caused by the sound transmission path 2 becomes “4α+3β”.
Accordingly, the cell multiplexer 10 receiving the cells includes a received-cell buffer, and controls the received-cell buffer to absorb the delay fluctuation 4α+3β of the cells caused by the sound transmission path 2. Thus, the cell multiplexer 10 can also absorb the delay fluctuation caused by the transmission path 1 by use of the received-cell buffer. The cell multiplexer 10 then reproduces the digital sound signal by executing a cell disassembly process and a sound expansion process on the cells whose delay fluctuation has been absorbed, and supplies the digital sound signal to the PBX 11. The PBX 11 converts the digital sound signal supplied from the cell multiplexer 10 to the analog sound signal, and supplies the analog sound signal to the telephone set 12. Consequently, the analog sound signal is transmitted from the telephone set 1 to the telephone set 12.
A cell multiplexer on a receiving end includes a received-cell buffer and controls the received-cell buffer so that the received-cell buffer can absorb the maximum delay fluctuation of cells caused by a sound transmission path, thereby enabling absorption of delay fluctuation of the cells caused by other sound transmission paths. In order to absorb a greater amount of delay fluctuation, the cell multiplexer on the receiving end controls a depth of the received-cell buffer to be deeper. As a result of deepening the depth of the received-cell buffer, transmission delay corresponding to the depth of the received-cell buffer occurs in a signal transmission.
On the other hand, a cell multiplexer on a transmitting end selects a route used for transmitting the cells by following exchange information set in a exchange management setting table and speed information set in a scheduler. In other words, a route through which the cells are transmitted is not predetermined in a case in which there exists a plurality of routes for transmitting the cells. Consequently, the cell multiplexer on the receiving end controls the received-cell buffer so that the received-cell buffer can absorb the maximum delay fluctuation caused by a sound transmission path among sound transmission paths possibly selected. For instance, as shown in FIG. 1, in a case in which the cell multiplexer on the transmitting end selects the sound transmission path 1, the cell multiplexer on the receiving end controls the received-cell buffer so that the received-cell buffer can absorb the delay fluctuation “α+3β” caused by the sound transmission path 2, even though the amount of the delay fluctuation caused by the sound transmission path 1 is “2α+β”. In such a case, the received-cell buffer included in the cell multiplexer on the receiving end causes extra transmission delay corresponding to an amount of delay fluctuation “2α+2β” in a transmission of the cells through the sound transmission path 1.
The transmission delay in a sound-signal transmission causes a sense of incongruity and decrease in sound quality in voice communication through a network. Thus, minimization of the transmission delay is desired in the sound-signal transmission in order to maximize the sound quality in the voice communication. However, by limiting a control of a received-cell buffer included in a cell multiplexer on a receiving end to a fixed setting for absorbing the maximum delay fluctuation caused by a sound transmission path, sound quality of a sound signal transmitted through a sound transmission path other than the sound transmission path causing the maximum delay fluctuation is decreased by the transmission delay corresponding to the maximum delay fluctuation caused by the received-cell buffer.